Such a drive train, which comprises an internal combustion engine that drives the drive wheels of the motor vehicle using a clutch and a manual gearbox, is known from practical experience. Here, the internal combustion engine is in control connection with an engine control unit, which adjusts the torque of the internal combustion engine corresponding to a torque set value signal preset at a set value signal input of the engine control unit. The torque set value signal can be predetermined directly by the driver using an electronic accelerator or by an electronic travel-stabilizing device. The manual gearbox is arranged in the drive train between the clutch and the drive wheels and its gear transmission ratio can be adjusted with the help of a motor-driven actuating device. The manual gearbox and the actuating device have a drive connection to an electronic transmission control system, which comprises a limit value signal output using which a limit value signal is output to the engine control unit in order to limit the torque of the internal combustion engine during the shifting operations. The shifting operations can thus be carried out such that they are largely unnoticed by the passengers of the vehicle.
The clutch is designed as a self-opening, not self-locking clutch, which is bent into a closed position by means of a clutch actuator against the restoring force of a plate spring. The clutch actuator has an electric motor as a drive, which is constantly energized when the clutch is closed, in order to apply the required closing force for the clutch. In order to keep the thermal power dissipation occurring in the motor winding as low as possible on average, the winding current is adjusted proportionally to the torque of the internal combustion engine, i.e., the winding current increases with the increasing torque. In the case of a low load of the internal combustion engine, this results in a corresponding lesser heating of the electric motor. The latter is dimensioned in such a way that a thermal overload of the electric motor is prevented in operating states occurring normally in practice. Nevertheless, under disadvantageous conditions, if e.g., in the case of high ambient temperatures, a long distance is covered continuously with maximum torque of the internal combustion engine, the permissible operating temperature of the electric motor is exceeded temporarily. This thermal load reduces the service life of the electric motor accordingly. This is disadvantageous especially since in the case of the breakdown of the electric motor, the clutch is arranged in the open position as a result of which the motor vehicle can no longer be moved by itself. The service life of the electric motor could indeed be increased by dimensioning it to be large enough so as to never be thermally overloaded even in the case of disadvantageous operating states occurring only seldom in practice. However, such an electric motor would be relatively large, correspondingly heavy and expensive.
A drive train for a motor vehicle is already disclosed in DE 198 06 497 C2, said drive train being foreign to the classification in question. In said drive train, the internal combustion engine drives the drive wheels using a conventionally designed frictional clutch. The frictional clutch can be adjusted with the help of an activation arrangement comprising a clutch pedal against the restoring force of a diaphragm spring from the open position into the closed position. By applying pressure on the clutch pedal, the driver of the motor vehicle can disengage the clutch, which is fully engaged in the resting state. The clutch has a clutch position sensor for detecting the closed position. Furthermore, slip sensors are provided for detecting the clutch slippage state. The clutch position sensor and the slip sensors are connected to a slip controller, which at the determination of a slippage state, limits a set value signal for the torque of the internal combustion engine, said set value signal being predetermined by the driver using an accelerator pedal. As stated in the patent specification, it is thus possible to dimension the frictional clutch and other components of the drive train to be lighter compared to a corresponding clutch without a slip controller. However, the disadvantage of the drive train is that the torque of the internal combustion engine is limited only after the clutch slippage has already occurred and the clutch can no longer transmit the required torque.